Our water heating stays on all the time, as it's a part of the air source heating system that we can't turn on/off independently of the radiator heating. As soon as the temp of the water drops to 46°, it heats up again to around 52°.
Ive been thinking that this is something that we need to take up with the landlord, to see if anything can be done about it. I am absolutely certain that for much of the year, it would be cheaper to boil a kettle for the little hot water that we use, though obviously showering would be difficult.
This coming summer, once we dont need any heating, I will put that to the test by turning the whole system off, and doing exactly that, just boiling a kettle, for our hot water.
I monitor our energy usage carefully, so should quickly see if it makes a difference.